Redox flow battery being a new electrochemical energy storage technology exhibits high energy conversion efficiency, flexible design, high energy storage capacity, flexible location, deep discharge, high safety, environmental friendly and low maintenance cost compared with other energy storage techniques. A number of applications have been considered, such as renewable energy storage for wind energy, solar energy etc., Emergency Power Supply, Standby Power System, peaking shaving as well as load leveling. Vanadium flow battery (VFB) has been considered as one of the most compelling electrochemical energy storage techniques due to its features like high safety, good stability, high efficiency, life longer than 15 years and low cost.
A membrane/separator, being one of the key materials of a VFB, is employed to prevent the cross mixing of the positive and negative electrolytes and compete the current circuit by transferring protons. The proton conductivity, chemical stability and ion selectivity of the membrane can directly affect the electrochemical performance and lifetime of VFB. Therefore, the membrane should possess a number of properties, including low active species permeability (high ion selectivity), low membrane area resistance (high ion conductivity), high physicochemical stability and low cost. The membranes most commonly used in VFB are perfluorosulfonic acid polymers such as DuPont Nafion® owing to their high proton conductivity and chemical stability. However, Nafion® membranes suffer from their extremely high cost, especially exhibit the disadvantage like low ion selectivity etc. when used for the VFB, which limits their further commercialization of VFB. Therefore, to develop a membrane with high ion selectivity, high physicochemical stability and low cost is of vital importance.
Currently, the membrane developed and studied for VFB application is ion exchange membrane consisted of the polymer with ion exchange groups. The ion exchange membrane can be divided into perfluorosulfonic ion exchange membrane, fluorosulfonic ion exchange membrane and non-fluorosulfonic ion exchange membrane. Because these membranes containing fluorosulfonic showed some problems such as low ion selectivity on vanadium ions and high cost, considerable effort had been gone into the development and characterization of non-fluorosulfonic ion exchange membrane, including sulfonated polyaryletherketone, polyether sulphone and polyether sulphone etc. The ion exchange groups are to transport protons and provide a bather to vanadium ions, and the main chain of polymer is to keep their mechanical stability. For the most non-fluorosulfonic ion exchange membrane, the induction of ion exchange groups can dramatically lower the chemical stability, which limits the life-time of membrane in VFB.
The membrane separation process is a process that the components of the raw material selectively transport the membrane to separate and refresh the raw material when there is a force difference (i.e. pressure difference, concentration difference and current difference etc.) between two sides of the membrane to achieve the purpose of separation or purification. The structure of the membrane is usually porous structure. The molecules with different size can selectively transport membrane based on the porous size of the membrane to achieve the purpose of separation or purification. The membrane most commonly used in the industry is usually prepared via the phase transition. The basic method is described as follows: the polymer solution is cast on a plate (i.e. glass plate). After evaporation of a moment, if necessary, the cast was immersed into non-solvent for the polymer to form the porous membrane. Various preparation parameters in this invention can affect the morphology and performance of the prepared membranes, such as the evaporation time of solvent, the concentration of the polymer solution and co-solvent etc. Different materials can be selectively separated via controlling the condition of forming the membrane and the pore size of the membranes.
In the case of the all vanadium flow battery, the vanadium ion and proton are both existed in the form of hydrated ion. The stokes radius of the V(VI) ion is about 2.5˜3A° [14]. Based on the formula of Stokes radius, it is concluded that there is an inverse relationship between the Stokes radius and the permeability parameter of the ion. However, the permeability parameter of proton [15] is far more than that of the vanadium. Therefore, the Stokes radius of the vanadium ion is far more than that of the proton.
      R    H    =                    k        B            ⁢      T              6      ⁢                          ⁢      π      ⁢                          ⁢      η      ⁢                          ⁢      D      
(kB is the Boltzmann constant, T is the Kelvin temperature, D is the ion permeability coefficient and η is the solution viscosity).
Based on the different stokes radius of between vanadium ions and protons, it could be possible to make protons freely transport across membrane and vanadium ions retained, realizing the function of ion exchange membranes for the VFB. This kind of membranes can meet the need of membranes via tuning the pore size without the introduction of the ion exchange groups, giving more choices of the membranes for the VFB application.